Along with miniaturization of a semiconductor device, the trend is moving into a shorter wavelength of the exposure light source and a higher numerical aperture (higher NA) of the projection lens, and at present, an exposure machine with NA of 0.84 has been developed, where an ArF excimer laser having a wavelength of 193 nm is used as a light source. As commonly well known, these factors can be expressed by the following formulae:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of the exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are coefficients related to the process.
For more shortening the wavelength and thereby obtaining higher resolution, studies are being made on an exposure machine where an F2 excimer laser having a wavelength of 157 nm is used as a light source. However, the lens material used for the exposure apparatus so as to realize shorter wavelength and the material used for the resist are extremely limited and therefore, it is very difficult to stabilize the production cost or quality of the apparatus and materials. This may lead to a failure in outfitting the exposure apparatus and the resist each assured of sufficiently high performance and stability within a required time period.
A so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between a projection lens and a sample has been conventionally known as a technique for enhancing the resolution in an optical microscope.
As for the “effect of immersion”, assuming that NA0=sin θ, the resolution and the depth of focus in immersion can be expressed by the following formulae:(Resolution)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 wherein λ0 is the wavelength of exposure light in air, n is the refractive index of the immersion liquid based on air, and θ is the convergence half-angle of beam.
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, when the projection optical system has the same NA, the depth of focus can be made n times larger by the immersion. This is effective for all pattern profiles and furthermore, can be combined with the super-resolution technology under study at present, such as phase-shift method and modified illumination method.
Examples of the apparatus where the effect above is applied to the transfer of a fine image pattern of a semiconductor device is described in JP-A-57-153433 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-7-220990.
Recent technical progress of the immersion exposure is reported, for example, in SPIE Proc., Vol. 4688, page 11 (2002), J. Vac. Sci. Tecnol. B, 17 (1999)) and SPIE Proc., Vol. 3999, page 2 (2000) and JP-A-10-303114 and International Publication No. 04/077158.
In the case of using an ArF excimer laser as a light source, pure water (refractive index at 193 nm: 1.44) is considered to be most promising as the immersion liquid in view of safety in handling as well as transmittance and refractive index at 193 nm. In the case of using an F2 excimer laser as a light source, a fluorine-containing solution is being studied from the aspect of balance between transmittance and refractive index at 157 nm, but a satisfactory solution in terms of environmental safety and refractive index has not yet been found. Considering the degree of immersion effect and the perfection of resist, the immersion exposure technique is expected to be most soon mounted on an ArF exposure machine.
Since the advent of a resist for KrF excimer laser (248 nm), an image fanning method called chemical amplification is used as an image forming method for a resist so as to compensate for sensitivity reduction caused by light absorption. For example, the image forming method by positive chemical amplification is an image forming method of decomposing an acid generator in the exposed area upon exposure to produce an acid, converting an alkali-insoluble group into an alkali-soluble group by using the generated acid as a reaction catalyst in the baking after exposure (PEB: Post Exposure Bake), and removing the exposed area by alkali development.
Various compounds have been also found for the acid generator that is a main constituent component of the chemical amplification resist composition, and a resist composition containing a specific sulfonium salt compound as the acid generator is disclosed in JP-A-10-232490 and JP-A-2003-195489.
JP-A-2004-271629 discloses a positive resist composition containing a polyhydroxystyrene-based resin and from 5 to 20 mass % of an acid generator with an attempt to realize high sensitivity, high resolution, good pattern profile and reduced line edge roughness in the patterning by X-ray, electron beam or EUV exposure.
However, in view of overall performance as a resist, it is actually very difficult to find out an appropriate combination of a resin, a photo-acid generator, an additive, a solvent and the like used for the pattern formation. In the formation of a fine pattern having as small a line width as 100 nm or less, the normal dry exposure and particularly the immersion exposure are faced with a tapering problem of the pattern profile due to refinement, or many insufficient points still remain, and improvement of line edge roughness (LER) is being required.